Physiological changes in pregnancy
Symptoms of pregnancy
Pregnancy leads to many symptoms which would be considered an illness or disease at other times. These symptoms are caused by hormonal changes which help create an optimum environment for the fetus and to support the increasing needs of the mother and child. Symptoms shown in pregnancy include anaemia, breathlessness, pelvic pain, swollen legs, increased urination, flushing, nausea, backaches, increased pigmentation, swollen or tender breast, and tiredness.
Effects of fetus to the respiratory system
There is an increased utilization of oxygen due to the fetus. (about 20% at term)
The expanding uterus limits the movement of the maternal diaphragm.
This leads to feelings of breathlessness
Compensatory changes in the respiratory system
l Increased tidal volume
n relaxin loosens the ligaments of the ribs allowing more outward movement, leading to a compensatory increase in tidal volume.
n Diaphragm excursion (movements) also increase.
n Residual volumes gradually decreases from the middle of the second trimester to the third trimester.
l Increased respiratory rate (controlled by the respiratory centre in the cerebral medulla oblongata)
n Progesterone increase the ventilatory centre’s sensitivity to CO2 and resets the CO2 activation level leading to increased ventilation. – ie less CO2 is needed to produce the same increase in ventilation.
n Progesterone also directly causes increased respiratory rate.
l Decreased resistance – due to relaxation of the smooth muscles of the airways caused by progesterone.
l Increased ventilation and blood flow results in increased gas transfer, which leads to an increase of PO2 and an decrease in PCO2
n This helps remove CO2 from the fetus as it increases the diffusion gradient.
n Increased PCO2 results in slight alkalosis, which is compensated by the excretion of bicarbonate and sodium.
Effects of the fetus on the cardiovascular system
There is increased capacity in the placenta, uterus, breasts, renal system and striated muscle to support the fetus and vasodilatation leads to increased vascular space.
There is an increase oxygen demand leading to an increase in RBC.
Partuition leads to blood loss.
Compensatory changes in the cardiovascular system
l Increased vasodilation.
n Progesterone causes the relaxation of smooth muscles and hence vasodilation.
n This leads to increased perfusion in the kidneys and lungs.
n This leads to underfilling of vessels and an initial drop in blood pressure.
n Increased peripheral perfusion also leads to flushing, ankle swelling and varicose veins.
l Increased blood volume and red blood cells
n There is a 40-50% increase in plama volume and a 20% increase in red blood cells, due to increased vascular space and metablic needs of the fetus.
n The unequal increase in blood plasma leads to hemodilution, leading to signs of anemia. Folate and iron deficiency may also lead to anaemia.
n Increase in blood plasma also reduces the impact of blood loss in delivery.
l There is a change in blood constituents
n The total amount of fibrinogen and clotting factors increase.
n The total amount of platelets also increases.
n This reduces the blood loss during delivery – however clotting time usually does not change during pregnancy.
n There is also a increase in white blood cells during and after partuition (giving birth)
l There is an increase in cardiac output (30-40%)
n This is primarily due to an increased stroke volume and partially through increased heart rate.
n The increased stroke volume occurs due to increased plasma volume, leading to increased venous return. This leads to increased stretch of the heart leading to stronger contractions (according to starling’s law)
n Oestrogen also increase cardiac muscle contractility and increase heart rate.
l The position and size of the heart changes
n The heart is shifted up and leftwards.
n There may be slight hypertrophy and larger appearance due to stretching.
l There are only slight changes in blood pressures.
n Increased cardiac output is compensated by the increase in vascular space through vasodilatation
n There is a slight dip in blood pressure due to underfilling mid pregnancy which corrects itself.
n Femoral pressures may increase due to mechanical factors.
l The growing fetus may compress the inferior vena cava causing a transient drop in cardiac output. This is usually compensated by increased vascular resistance and heart rate.
Effects of the fetus on the renal system
There is a reduction of plasma osmolarity due to the excretion of bicarbonates and exhalation of carbon dioxide.
There is a doubling in blood flow and a 50% increase in glomerular filtration rate.
There is an increase in the size of the vascular compartment.
Changes in the renal system
l Glomerular filtration rate increases
n This is due to the increased plasma volume, decreased vascular resistance and increased cardiac output leading to increased perfusion to the kidney.
n The decreased oncotic pressure further increases filtration rate
n There is reduced creatinine levels, and increased urination frequency in pregnant women due to this.
l There is an increase in water reapsorption to compensate for the increased vascular size.
n The juxtaglomerular cells activate the rennin-angiotensin-aldosterone system leading to increased sodium reabsorption and water uptake.
n Progesterone rests the activation levels of osmoreceptors such that thirst and ADH secretion is not inhibited due to the decreased osmolarity.
n Vasodilation means that atrial stretch receptors are not activated so the rennin angiotensin system is not inhibited.
l The kidneys cannot reabsorbed nutrients at the same rate as the filtration rate leading to the presence of protien and glucose in urine.
Effects of the fetus on the gastro-intestinal system
The growing uterus displaces the GI tract
Hormones of pregnancy may reduce the motility of the GI tract.
Effects of the fetus on the metabolism
There is an increased need for nutrients.
Compensatory changes in metabolism
l There is an increase in protein and fat metabolism for growth of the fetus and uterus.
l There is an increased insulin resistance leading to higher blood sugar.
Breast development
l Oestrogen causes the development of mammary ducts
l Progesterone causes the development of breast lobules
l Prolactin rises steadily throughout pregnancy and stimulates milk production, however high levels of oestrogen inhibits this.
l Suckling stimulates oxytocin mediated milk ejection and continued prolactin secretion.
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